1. Field of the Invention
The present invention relates to a power supply circuit for supplying a voltage to a power amplifier, and particularly to a power supply circuit which is capable of operating with low noise and low power consumption by optimizing an internal current thereof.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to secure precision of an output signal and operate with low power consumption. In such a communication device, a transmission circuit capable of operating with low distortion and high efficiency is used. Hereinafter, conventional transmission circuits will be described.
One of the conventional transmission circuits is, for example, a transmission circuit which uses a modulation method such as a quadrature modulation method to generate a transmission signal (hereinafter, referred to as a quadrature modulation circuit). Since the quadrature modulation circuit is well known, a description thereof will be omitted. A conventional transmission circuit, which is smaller in size and operates more efficiently than the quadrature modulation circuit, is, e.g., a transmission circuit 500 shown in FIG. 13. FIG. 13 is a block diagram showing an exemplary structure of the conventional transmission circuit 500. In FIG. 13, the conventional transmission circuit 500 comprises a signal generator 501, an output terminal 502, a power supply circuit 503, a power amplifier (PA) 504 and a power supply terminal 505.
In the conventional transmission circuit 500, the signal generator 501 generates an amplitude signal and an angle-modulated signal. The amplitude signal is inputted to the power supply circuit 503. The power supply circuit 503 supplies the power amplifier 504 with a voltage which is controlled in accordance with the inputted amplitude signal. The power supply circuit 503 typically supplies the power amplifier 504 with the voltage which is proportional to a magnitude of the inputted amplitude signal. Here, the power supply circuit 503 is supplied with a DC voltage from the power supply terminal 505.
The angle-modulated signal is inputted to the power amplifier 504. The power amplifier 504 amplifies the angle-modulated signal by using the voltage supplied from the power supply circuit 503, and outputs the amplified signal. The signal amplified at the power amplifier 504 is outputted from the output terminal 502 as a transmission signal. Thus, in the conventional transmission circuit 500, the power supply circuit 503 controls, in accordance with the amplitude signal, the voltage to be supplied to the power amplifier 504, and this allows the power amplifier 504 to operate with low power consumption. The transmission circuit 500 as above is called a polar modulation circuit.
Another one of the conventional circuits is a transmission circuit 600 shown in FIG. 14 which is disclosed by Japanese National Phase PCT Laid-Open Publication No. 2004-530313 (hereinafter, referred to as Patent Document 1). FIG. 14 is a block diagram showing an exemplary structure of the conventional transmission circuit 600. In the conventional transmission circuit 600 shown in FIG. 14, bias control circuits (power supply circuits) 660A and 660B control, in accordance with gain information 628 from a gain control circuit 630, bias currents of RF circuits such as an IF buffer 642, a mixer 644, an RF VGA 546, a power amplifier (PA) 650 and the like. To be specific, the conventional transmission circuit 600 controls, in accordance with a signal gain thereof, the bias currents of the RF circuits such as the power amplifier 650 and the like, thereby enabling the RF circuits to operate with lower power consumption.
However, although the conventional transmission circuit 500 is capable of enabling the power amplifier 504 to operate with low power consumption, the transmission circuit 500 does not realize saving of power consumption of the power supply circuit 503. In particular, when a power level of the power amplifier 504 is low, power consumption of the entire system is small, and a proportion of the power consumption of the power supply circuit 503 in the power consumption of the entire system is relatively large. Thus, the conventional transmission circuit 500 has a problem that when the power level of the power amplifier 504 is low, the power consumption of the power supply circuit 503 causes efficiency deterioration of the entire transmission circuit 500.
Since an internal current of the power supply circuit 503 does not depend on the power level of the power amplifier 504, it seems likely that the power saving of the power supply circuit 503 is realized by reducing the internal current of the power supply circuit 503. In this case, however, there is a problem that noise is increased when the internal current of the power supply circuit 503 is reduced. Thus, the conventional transmission circuit 500 has a problem that it is difficult to reduce both the noise and power consumption of the power supply circuit 503.
Also, the conventional transmission circuit 600 only realizes power saving of the RF circuits such as the power amplifier 650 and the like, and does not realize power saving of the power supply circuits 660A and 660B. Accordingly, the conventional transmission circuit 600 also has a problem that the power consumption of the power supply circuits 660A and 660B causes efficiency deterioration of the transmission circuit 600. Further, the conventional transmission circuit 600 also has a problem that it is difficult to reduce both noise and power consumption of the power supply circuits 660A and 660B since the noise is increased when an internal current of the power supply circuits 660A and 660B is reduced.